1. Field of the Invention
The present invention relates to hearing prosthesis and to sound processing devices and methods associated with hearing prosthesis. In particular, the present invention relates to an apparatus and method of envelope detection that is simple to implement in both analog circuitry or digital signal processing and assists cochlear implant recipients to better perceive changes in the amplitude of speech than is currently the case. Furthermore, the invention relates to an apparatus and method for enhancing the pitch cue of an audio signal perceived by a cochlear implant recipient.
2. Background of the Invention
In many people who are profoundly deaf, the reason for deafness is absence of, or destruction of, the hair cells in the cochlea which transduce acoustic signals into nerve impulses. These people are unable to derive suitable benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because there is damage to or absence of the mechanism for nerve impulses to be generated from sound in the normal manner.
It is for this purpose that cochlear implant systems have been developed. Such systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve. U.S. Pat. No. 4,532,930, also in the name of the applicant and the contents of which are incorporated herein by reference, provides a description of one type of traditional cochlear implant system.
Typically, cochlear implant systems have consisted of essentially two components, an external component commonly referred to as a processor unit and an internal implanted component commonly referred to as a receiver/stimulator unit. Traditionally, both of these components have cooperated together to provide the sound sensation to a user.
The external component has traditionally consisted of a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts the detected sounds, particularly speech, into a coded signal, a power source such as a battery, and an external transmitter coil.
The coded signal output by the speech processor is transmitted transcutaneously to the implanted stimulator/receiver unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter coil which is positioned to communicate with an implanted receiver coil provided with the stimulator/receiver unit.
This communication serves two essential purposes, firstly to transcutaneously transmit the coded sound signal and secondly to provide power to the implanted stimulator/receiver unit. Conventionally, this link has been in the form of a radio frequency (RF) link, but other such links have been proposed and implemented with varying degrees of success.
The implanted stimulator/receiver unit traditionally includes a receiver coil that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlear electrode assembly which applies the electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound.
Traditionally, the external componentry has been carried on the body of the user, such as in a pocket of the user's clothing, a belt pouch or in a harness, while the microphone has been mounted on a clip behind the ear or on the lapel of the user.
More recently, due in the main to improvements in technology, the physical dimensions of the speech processor have been able to be reduced allowing for the external componentry to be housed in a small unit capable of being worn behind the ear of the user. This unit allows the microphone, power unit and the speech processor to be housed in a single unit capable of being discretely worn behind the ear, with the external transmitter coil still positioned on the side of the user's head to allow for the transmission of the coded sound signal from the speech processor and power to the implanted stimulator unit. It is envisaged that with further technological advancements the system components will be able to be fully implanted within the head of the recipient, providing a totally invisible device.
As the ability to perceive sound is of fundamental importance to cochlear implant recipients, the ability to reproduce sound and the percepts of speech via electrical stimulation using a cochlear prosthesis is one of the major challenges of this technology. It is the speech processor that provides the link between the acoustic representation of speech and the pattern of neural discharges which the stimulator of the implant is able to induce, and which the recipient experiences as hearing sensations. Many speech-processing strategies such as Continuous Inter-leaved Sampling (CIS), and those based on spectral maxima SPEAK and ACE, have been proposed to improve the quality of the sensation as perceived by the recipient, in a number of different sound environments.
These strategies utilise envelope detection for processing the output of a series of filters, however a disadvantage of such systems is that the output of the envelope detector typically includes a large amount of ripple and/or the desired envelope becomes excessively smeared out. This has the disadvantage of adversely affecting the temporal cues that are important in consonant perception. Other implementations of the strategies may result in the output having a ripple which is aliased causing the stimulation amplitude to vary with a frequency which is not present in the input sound. Such a ripple can modulate at a beat frequency which can give erroneous pitch cues to the implant recipient.
Another prior system called quadrature envelope detection, although producing an envelope which substantially contains no ripple and is not smeared out, has the disadvantage that it is complex and requires twice as many band pass filters as there are frequency channels which results in additional cost and complexity. Furthermore, the need to provide the function of squaring and square root operations is generally not practical in analogue circuitry.
In normal hearing, the inner hair cells only respond to movement of the basilar membrane in one direction. They tend to fire in phase with the basilar membrane response. This is known as “phase-locking”; it preserves the timing content of the basilar membrane response, and it is believed to be important for pitch perception. At high stimulation rates, the fine timing content generally has not been taken into account and therefore implant recipients have not been able to adequately perceive pitch in an audio signal. The present invention substantially preserves the fine timing content of the band-pass filter outputs, and provides an additional pitch cue to the cochlear implant recipient. It requires high stimulation rates.
The present invention is therefore related to improving the manner in which an audio signal is processed so that the quality of sound reproduced via the electrical stimulation is substantially maintained.